The present invention relates to display and projection systems using multiple micro electro mechanical systems (MEMS) as spatial light modulators (SLMs). More specifically the invention overcomes restrictions in the design of superposition systems rooted in the chirality (handedness) of MEMSs (e.g. digital mirror devices (DMD) from Texas Instruments) according to the state of the art. More specifically the present invention uncovers a superposition method using a pair of stereo-isomeric MEMSs (right-handed and left-handed stereo-isomeric topologies).
All display systems which use more than one SLM (e.g. standard 3-chip systems for simultaneous RGB color generation or true parallel 2-chip stereo projection devices generating left and right eye information simultaneously) have to use at least one superposition layer. The spatially modulated ON-light of one SLM transmits this layer and the ON-light of a second SLM is reflected at this layer which thus combines both ON-lights into a common optical path.
A single reflection always means that the image is transformed to its mirror image. Thus one of the two ON-beams which have to be superposed has to carry this image information mirror-symmetrically with respect to the other. In general this can easily be realized by electronically transforming the respective image information. E.g. in reflective liquid crystal on silicon (LCOS) modulator systems, the spatial light modulation is controlled by the polarization state of the light whereby ON-beams and OFF-beams have the opposite polarization. Both beams are oriented normal to the chip surface. LCOS (and LCD) modulators therefore have a symmetry axis with respect to their overall performance. Obviously, MEMSs can also be addressed mirror symmetrically. However, in DMD displays the spatial light modulation is controlled by the direction of the light beams. This requires more sophistication in the light guidance, as both the IN-light and OFF-light are not normal to the surface of the modulator; only the optical ON-axis is normal to the chip surface (compare FIG. 3A).
In addition, MEMSs of the state of the art (e.g. DMDs by Texas Instruments U.S. Pat. No. 5,600,383) do not show any axes of symmetry considering their overall operation. Only the “ON” beam is reflected perpendicularly to the modulator surface. The incident beam, however, is perpendicular to the mirror deflection axis (the deflection axis is normal to the plane of incidence), which is rotated by 45° to the image raster. On the other hand, the incidence angle is twice the deflection angle of a single mirror (e.g. 24°) (See FIG. 1A). This asymmetric overall performance has profound impact on system designs for multiple imagers.
A DMD with a 4×3 matrix is shown in FIG. 1B (the modulator is shown as an example of a landscape format oriented rectangular array of modulator elements with w columns and h rows; this corresponds to a usual width/height ratio and orientation of current DMD modulators). Single mirrors (17) rotate around a deflection axis which has an angle of 45° relative to the raster image. In FIG. 1B the single mirror deflection axes (21) are visible after “removing” the mirrors. The DMD corresponds to the topological type built by Texas Instruments (U.S. Pat. No. 5,600,383) according to the state of the art. While the rectangular raster image itself shows internal symmetry, the DMD is not a symmetrical structure because the orientation of the mirror deflection axis has to be taken into account. For overall operation no internal symmetry exists. Not by rotation, but only by a reflection the currently produced “L”-topology is “converted” into a “R”-topology (compare FIG. 3A-C). Due to their rectangular shape and to the orientation of the mirror deflection axes, which are rotated 45° to the image raster, MEMS of the state of the art show stereo-isomery (chirality, handedness). Handedness is characterized by the existence of two different topologies which are mirror symmetric and cannot be transformed into each other by rotation. The lack of a right-handed (R)DMD has profound impact on multiple chip design.
In all 2-chip or 3-chip DMD projection devices according to the state of the art the L(DMD) modulators are positioned such that their modulator element arrays are inclined by 45° relative to the plane of incidence (POI) of the superposition layer (see FIGS. 2, 9). In general, the mirror deflection axes are oriented parallel to the POI, which facilitates light guidance to and fro the modulators, with the IN- and OFF-lights being in a plane different from the POI of superposition (compare FIGS. 2, 9 wherein all IN-light is directed from below the superposition POI and all OFF-light is directed to a common dump above this plane). This design also allows for a common split- and combine system where the dichroic layers are used both to feed the DMDs with color-split light and to superpose the spatially modulated ON-light of the three DMDs into a common ON-beam.
However, such a rotated positioning of the DMDs requires additional actions for a successful superposition. While the shape of the ON-light of one DMD, which transmits the superposition layer remains unchanged, the shape of the ON-light of a second DMD which is reflected at the superposition layer is transformed to its mirror-image; this would result in a mismatch of the superposition (cause incomplete overlap; e.g. Bausenwein and Mayer U.S. application Ser. No. 11/716,649). Therefore, two principle ways to correct this are known to the state of the art: either the use of an additional reflecting surface for the transmitting ON-beams (equalizing the number of reflections (comp. Kavanagh and Fielding U.S. Pat. No. 5,638,142 entitled: “spatial light modulator system including a plurality of tiltable mirror devices and reflective means for equalizing the number of reflections from the tiltable mirror devices”; Fielding et al., U.S. Pat. No. 6,250,763 FIG. 3; Fielding U.S. Pat. Nos. 6,276,801; 6,631,993; Fielding GB 2 291 978) or the use of an additional reflecting surface for the reflecting ON-beams, which makes the difference of the number of reflections of the two superposed ON-beams an even number (in general, this difference equals 2); e.g. Fielding U.S. Pat. No. 6,250,763 page 7 line 31: “It will be appreciated that as the red and blue spatially modulated light undergoes two reflections prior to being recombined to form the output white spatially modulated light beam, it is not necessary to provide a further reflector in the green light path as shown in FIG. 3”).
In trichroic prism assemblies (TPA) according to the state of the art (FIG. 9) the IN- and ON-beams of the red and blue DMDs are reflected twice; whereas the IN- and ON-light of the green DMD transmits the superposition layer (see FIG. 9; prior art in U.S. Pat. No. 7,396,132). We have uncovered that the need for adjusting the number of reflections when using at least two (L)DMDs can be resolved; however this is achieved at the cost of increased complexity in the light guidance system (Bausenwein and Mayer U.S. Pat. No. 7,466,473). The use of a stereo-isomeric pair of an (L)DMD and its isomeric counterpart (R)DMD would allow for a liberated and simplified system design. Without having to adjust the number of reflections, the design of multi-DMD superposition arrangements benefits from facilitations of light guidance. This is what we uncover in our new superposition method.